The present invention generally relates to noise reduction systems for video signals, and more particularly to a noise reduction system for reducing noise in a video signal which is recorded on and reproduced from a recording medium by subjecting a recording video signal to a pre-emphasis in a vertical direction of a picture and in a direction oblique to the vertical direction in a recording system at the time of a recording and by subjecting a reproduced video signal to a de-emphasis complementary to the pre-emphasis in a reproducing system at the time of a reproduction.
Conventionally, pre-emphasis and de-emphasis circuits are known for reducing noise in a video signal which is recorded on and reproduced from a recording medium. In a recording system, a recording video signal is subjected to a pre-emphasis in the pre-emphasis circuit, and a reproduced video signal is subjected to a de-emphasis in the de-emphasis circuit of a reproducing system. In the case of a frequency modulated video signal, the signal-to-noise (S/N) ratio is poor for high frequency components of the frequency modulated video signal. However, the pre-emphasis circuit can emphasize the high frequency components of the recording frequency modulated video signal before the recording. At the time of the reproduction, the de-emphasis circuit de-emphasizes (attenuates) the emphasized high frequency components of the reproduced frequency modulated video signal so as to obtain the original frequency modulated video signal. Therefore, it is possible to record and reproduce the high frequency components of the video signal with a satisfactory S/N ratio by use of such pre-emphasis and de-emphasis circuits. Transversal filters are generally used for the pre-emphasis and de-emphasis circuits.
According to one conventional noise reduction system, the emphasis is carried out by subtracting from or adding to an information which is related to a predetermined point on a predetermined horizontal scanning line in a reproduced picture a plurality of information related to a plurality of points which are on the predetermined horizontal scanning line and are positionally leading or lagging (advanced in time or lagging in time) with respect to the predetermined point. By carrying out such pre-emphasis and de-emphasis, it is possible to average high frequency noise in the horizontal direction of the picture, and it is accordingly possible to improve the S/N ratio of the video signal. In the present specification, such an emphasis will be referred to as a "horizontal emphasis".
According to the above described conventional noise reduction system, the noise can be reduced satisfactorily in the case where the noise is in the form of a vertical line which is long in the vertical direction of the picture and is narrow in the horizontal direction of the picture, for example. The noise can be reduced satisfactorily in this case because the frequency of the noise in the horizontal direction is high. On the other hand, in the case where the noise is in the form of a horizontal line which is long in the horizontal direction of the picture and is narrow in the vertical direction of the picture, the frequency of the noise in the horizontal direction is low. For this reason, the noise reducing effect is poor in this case, and there is a problem in that the noise in the form of the horizontal line cannot be reduced satisfactorily.
Hence, other noise reducing systems were previously proposed in a U.S. Pat. No. 4,607,285 and a U.S. patent application Ser. No. 634,407 filed July 25, 1984 (allowed but patent number not yet known) in which the assignee is the same as the assignee of the present application. According to these previously proposed noise reduction systems, the emphasis is carried out by subtracting from or adding to an information which is related to a predetermined point in a reproduced picture a plurality of information related to a plurality of points which are a specific number of horizontal scanning lines before the predetermined point. By carrying out such pre-emphasis and de-emphasis, it is possible to average high frequency noise in the vertical direction of the picture, and it is accordingly possible to improve the S/N ratio of the video signal In the present specification, such an emphasis will be referred to as a "vertical emphasis".
According to the above described previously proposed noise reduction systems, the noise can be reduced satisfactorily in the case where the noise is in the form of a horizontal line which is long in the horizontal direction of the picture and is narrow in the vertical direction of the picture, for example. The noise can be reduced satisfactorily in this case because the frequency of the noise in the vertical direction is high. On the other hand, in the case where the noise is in the form of a vertical line which is long in the vertical direction of the picture and is narrow in the horizontal direction of the picture, the frequency of the noise in the vertical direction is low. For this reason, the noise reducing effect is poor in this case, and there is a problem in that the noise in the form of the vertical line cannot be reduced satisfactorily.
Generally, a noise reduction system of a magnetic recording and reproducing apparatus reduces the noise by assuming that the noise is random noise. However, the video signal recorded on the recording medium by the magnetic recording and reproducing apparatus has strong correlation in the horizontal direction of the picture and in the vertical direction of the picture. Furthermore, the video signal recorded on the recording medium also has strong correlation in terms of frames, that is, an information related to a predetermined point in the picture is correlated between successive frames. Accordingly, it may be regarded that the noise also has correlation.
For this reason, when the vertical emphasis is carried out in the previously proposed noise reduction systems, the noise becomes arranged in the vertical direction of the picture due to the correlation of the picture element data in the vertical direction of the picture, and the vertically arranged noise in the reproduced picture gives the viewer an impression as if it were raining. The viewer is used to seeing noise in the horizontal direction but is not used to seeing such noise in the vertical direction, and there is a problem in that such noise in the vertical direction is conspicuous to the viewer.
On the other hand, when designing the noise reduction system so as to improve the S/N ratio of the video signal, it is desirable that a recording medium recorded with the video signal on an existing magnetic recording and reproducing apparatus is compatibly playable on a magnetic recording and reproducing apparatus comprising the noise reduction system and vice versa.